Protective system



June 10, 1952 YQNKERS 2,600,149

PROTECTIVE SYSTEM Filed Aug. 50, 1949 2 SHEETSSHEET l 64 I; l fl i is II 65' L I I f INVENTOR. 5 Edward 7% j/Ollkl's E. H. YONKERS PROTECTIVESYSTEM June 10, 1952 2 SHEETS-SHEET 2 Filed Aug. 30, 1949 EQ &

m AM MW my a d 4w 6 M w i W Patented June 10, 1952 PROTECTIVE SYSTEMEdward H. Yonkers, Chicago, Ill., assignor to Joslyn Mfg. and SupplyCompany, Chicago, 111., a corporation of Illinois Application August 30,1949, Serial No. 113,069

14 Claims. 1

The present invention relates to a protective system or protectivedevices for electrical systerns and apparatus, and more particularly toprotective devices for protecting power transmission and distributionlines as well as station or other equipment from damage occasioned bylightning and other dangerous over-voltage surges upon the line.

This application is a continuation in part of copending applicationSerial No. 583,499 filed March 19, 1945, now abandoned.

In general, protective devices of the character mentioned may properlybe termed voltage limiting devices in that their purpose is to preventthe voltage between an energized current carrying conductor and groundfrom exceeding a predetermined dangerous level. The assigned functionsof such devices are threefold: First, they must act to provide a lowimpedance path between the energized conductor and ground as quickly aspossible after the voltage applied to the terminals of the deviceexceeds a predetermined dangerous'level. Second, the device must permitsurge currents to flow between the energized conductor and ground with apotential drop across the device which does not exceed the protectivelevel. Third, the device must eliminate the low impedance path betweenthe energized conductor and ground as soon as possible after the voltagesurge disappears or dies out so that a minimum of system or followcurrent is permitted to flow between the conductor and ground.Practically all commercially availabl protective devices capable ofhandling lightning and other dangerous over-voltages appearing upon apower line conductor employ one or more spark gaps or an isolating sparkgap in series With valve mate'- rial or a second spark gap housed withinan arc extinguishing chamber of some sort. There are on the market todaytwo well known types of such protective devices generally referred to asvalve type arresters and expulsion type arresters. The valve typearresters employ what is generally termed as valve material to limit thefollow current. This valve material has peculiar characteristics in thatits impedance changes in accordance with the voltage applied. For highvoltages the impedance is relatively low, whereas for low voltages theimpedance is fairly high, thereby acting as a valve to shut off the60-cycle follow current following a break-down of the gaps associatedwith the arrester. The expulsion arrester, on the other hand, comprisesgenerally an arc extinguishing chamber enclosing a pair of electrodesdefining an arc gap. Generally the above-described time-lag effect.

expulsion type arrester is provided with an arc confining insulatingmaterial from which gas is evolved when subjected to the heat of thearc, which gas aids in rapidly extinguishing the are following thedisappearance of the surge voltage.

Both valve type arresters and expulsion type arresters are characterizedby a time lag before they spark over when a transient potential isapplied thereto. More specifically, the impulsevolt-time characteristicof all known protective devices rises rapidly in the short time regionof the characteristic and approaches infinity as the time of voltageapplication approaches zero. That is to say, in the limiting conditionlightning arresters require a voltage approaching infinity to causeflash over as the time of application of this voltage approaches zero.

Since the starting or are initiating voltage of a protective device ispermitted to appear across the terminals of the device, it obviously isalso permitted to reach the apparatus or circuit which the device isintended to protect. One of the basic problems in the development oflightning protective devices, therefore, is that of keeping the time-lagof the device as low as possible, or stated otherwise, to speed up thestarting of all of the elements of the device which exhibit the Theimpulsefailur characteristic of insulation also exhibits an inherenttime-lag. However, it is not necessarily of the same shape as that ofthe protective device, and it may vary widely under difierent serviceconditions. It is imperative, therefore, to maintain a liberal marginbetween the starting characteristic of a protective device and theimpulse-failure characteristic of the insulation associated withapparatus or electrical circuits which the protective device is designedto protect. In general the voltage at which a protec tive device may beset to spark over or start operating'v is determined by the generatedvoltage of the system to which the protected apparatus and circuits asWell as the protective device are applied. In other words, theprotective device must work rapidly when a transient or surge voltageexceeding a certain threshold value is applied to its terminals, but itmust not operate on harmless system over-voltages which approach but donot exceed this threshold level.

Both valve type and expulsion type arresters or protective devices arecommonly equipped with so-called series isolating gaps. The purpose ofsuch gaps is to minimize the flow of system current through the deviceduring inactive periods and many times to eliminate the system voltageunder normal conditions from the arresters to prevent tracking in thecase of expulsion arresters and to eliminate leakage current through thevalve element in the case of valve arresters. Such gaps conventionallycomprise two or more spaced apart electrodes which exhibit the abovedescribed time-lag efiect wherein the voltage increases steeply in theshort time region of the impulse volt-time characteristic. All gaseousdischarge devices including isolating gaps, valve elements and expulsiontype are extinguishing units exhibit time-lag characteristics, and Wheretwo or more discharge paths provided by such elements are connected inseries the time-lag effects are more or less additive, with the resultthat the overall time required to start such a device or to initiate thesurge by-passing function of such device may be too great to provide theprotection when most needed, i. e., when a steep wave front transient oflarge magnitude appears across the terminals of the protective device.

Accordingly it is an object of the present invention to provide animproved arrangement for reducing the starting time of protectiveapparatus of the character described.

It is another object of the present invention to provide improvedfacilities for decreasing the overall sparkover time of a protectivedevice employing a plurality of discharge paths in series.

It is a further object of the present invention to provide a protectivedevice in the form of a reversing network which distinguishes betweenordinary system voltages and surge voltages to l6 extent of causing amajor change in voltage distribution across the discharge path of theprotective device when the system voltage changes to a surge voltage andvice versa.

Still another object of the present invention resides in the provisionof protective apparatus of the character described wherein differentsurge drain circuits having different over-all impulse volt-timecharacteristics are provided for limiting the rise of abnormal transientvoltages of different magnitudes and having different transientcharacteristics.

A still further object of the present invention is to provide, inconjunction with a series connected multi-discharge path protectivedevice an auxiliary impedance network having the function ofsimultaneously developing across each discharge path a voltage whichexceeds a major fraction of the voltage applied to the terminals of theapparatus at the instant that breakdown of the arc path should occur.

It is another object of the present invention to provide a multi-gapdischarge path protective apparatus wherein a minor fraction of thesystem voltage normally appearing across the terminals of the apparatusis applied to each of the end discharge paths of the apparatus andwherein these voltages are each increased to a major fraction of theterminal voltage upon the application of a transient voltage to theterminals of the apparatus.

It is another object of the present invention to provide protectiveapparatus of the character described which affords completely reliableprotection with respect to any and all forms of dangerous voltage waveswhich may appear upon a power line conductor with a minimum ofdisturbance to the power service.

Further objects and advantages of the present invention will becomeapparent as the following description proceeds, and the features ofnovelty which characterize the invention will be pointed out withparticularity in the claims annexed to and forming a part of thisspecification.

For a better understanding of the present invention reference may be hadto the accompanying drawings, in which:

Fig. l is a schematic diagram illustrating generally the protectivedevice of the present invention;

Fig. 2 is an elevational view, partly in section. of a commercialembodiment of a protective device as schematically illustrated in Fig.1;

Fig. 3 is an enlarged perspective view of one element of the protectivedevice of Fig. 2; and

Fig. 4 illustrates an enlarged section of a portion of Fig. 2 showing amodification of the present invention.

Referring now to the drawings and specifically to Fig. 1 of thedrawings, the improved protective device of the present inventiongenerally designated at I2 is illustrated as comprising a pair ofterminals [0 and II. Such protective devices are generally connectedbetween a device or circuit to be protected, such as a high voltagetransmission line and ground. As indicated in Fig. l, the terminal Illof the protective device 12 is connected to a transmission line [3 whilethe terminal H is connected to ground, generally indicated at M. Thusthe potential difference between the transmission line or conductor l3and ground 14 appears across the terminals [0 and II at all times. It isthe function of the protective device l2 to prevent the voltage acrossthe terminals I0 and II from rising to an abnormally high and dangerousvalue.

In accordance with the present invention, the protective device l2between the terminals Ill and H comprises what is termed hereinafter asa reversing network, including a plurality of discharge paths I5a, [6aand I1, which bridge the terminals in and II in series. The two enddischarge paths [5a and Mia are each provided between the electrodes[Sb-15c and l5b-|6c of arrester units 15 and [6, respectively,designated schematically by dotted lines in Fig. l of the drawings.These arrester units [5 and [6 may comprise any conventional type ofarrester units. Thus the arrester units I5 and [6 might comprise eitherexpulsion type arresters or valve type arresters as mentioned above.Preferably the arresters l5 and I6 are of the improved expulsion typedisclosed in Pittman Patents 2,336,420 and 2,418,791 granted December 7,1943, and April 8, 1947, respectively.

It will be understood that the impedance of each of the three identifieddischarge paths 15a, Ilia and [1 before sparkover is comprised of ashunt capacitance component schematically indicated as I5d, Hid and I'Idrespectively. In the event that there is no leakage current across thegaps, the only impedance presented before sparkover is thatschematically designated by the capacitors I5d, [6d and lid. In theevent of leakage current prior to sparkover, these discharge paths willalso present a resistive shunt impedance, which is designated in dottedlines as [5e and lfie in Fig. 1 of the drawings. This resistiveimpedance might be sufliciently low in the case of a valve arrester tobe important in determining the total shunt impedance of the dischargepath, Whereas in the case of a discharge path such as an air gap I 1under dry conditions, this resistive impedance is substantially infiniteand is negligible from the standpoint of the shunt impedance.Consequently in the case of the discharge path II no shunt resistance isindicated since this path is assumed to be an air gap. In the absence ofthe facilities described below forming an important part of the presentinvention, the terminal voltage as measured at the terminals l and H ofthe device l2 would necessarily divide between the three discharge pathsin accordance with the relative impedances thereof. As a result, thedischarge paths tend to spark over in succession during non-overlappingintervals such that the time-lag effects of the paths are cumulative andretard the complete sparkover between the terminals l0 and II when atransient voltage appears across these terminals.

For the purpose of more nearly equalizing the voltages individuallydeveloped across the three discharge paths l5a, |6a and I1 upon theoccurrence of a transient voltage between the terminals ill and M, anauxiliary impedance network is connected in circuit with the threedischarge paths, the component impedances of which are so proportionedas normally to maintain a minor fraction of the terminal voltage acrosseach of the end discharge paths |5a and I60, and to increase thesevoltages in response to the appearance of an abnormal transient voltagebetween the terminals l0 and to at least a major fraction of theterminal voltage. Specifically, this auxiliary impedance networkcomprises resistors l9 and respectively shunting the arrester units l5and I6 including the discharge paths |5a and |6a, and two relativelylarge capacitors 2| and 22 connecting the discharge paths I50, and |6arespectively directly across the terminals l0 and The total shuntresistance across the discharge path I5a'is, therefore, the combinedresistance afforded by the parallel resistors l9 and |5e. As wasmentioned above, in some cases the ohmic impedance of the resistor |5eis so high that it can be neglected in determining the shunt resistiveimpedance of the discharge path |5a, and only the auxiliary resistor |9need be considered. This is similarly true with respect to the resistor20 and the associated resistor lfie. In the event that the arresters l5and I6 are valve type arresters the resistance value of the resistor |6ebecomes important. As has been mentioned above, since the gap I1 is anair gap, the shunt resistance is of such high value as to have noappreciable eifect upon the operation of the network as a whole, andconsequently it has not been indicated even in dotted lines. The shuntcapacities, however, |5d, lid and H11, although very small in the caseof ordinary gaps, are shown in dotted lines in the drawings, since undercertain frequency conditions from the standpoint of reactive impedancethey become important. Although the auxiliary impedances l9 and 20 havebeen specifically indicated as resistors. it will become apparent as thefollowing description proceeds that these auxiliary impedances mightalso comprise, under certain conditions, inductances, and it is intendedin the appended claims to cover impedances including resistance as wellas inductance which will perform the function set forth above andelaborated upon in greater detail hereinafter.

Further in accordance with the present invention and before consideringthe operation of the protective apparatus |2 of the present invention,the capacitor 2| is designed to have a reactive impedance which is onlya fraction of the reactive impedance provided by the capacitor l5d. Thissame fractional relationship also holds with respect to the reactiveimpedances of capacitors 22 and Hid. On the other hand, the effective resistance in shunt with each of the discharge paths |5a and Mia, which isthe parallel impedance afforded by the resistors |5e and I9 in the caseof discharge path lie, and "5e and 20 in the case of discharge path l6a,has a resistive impedance which is small compared with the reactiveimpedance of the capacitors 2| and 22 at the system or GO-cyclefrequency. More specifically, the capacitance of the capacitor |5d ispreferably of the order of one-twentieth that of the capacitance ofcapacitors 2 and the effective shunt resistance of the parallel arrangedresistors |5e and I9 provides a resistive impedance of approximatelyone-tenth the reactive impedance of the capacitor 2| at the system or60-cycle frequency. The same impedance relationships should also prevailwith respect to the circuit components comprising elements 22, IN, lieand 20.

As has been pointed out above, in many cases the resistors |5e and |6emay be completely eliminated from the calculations, since they arenegligible in so far as the eifective parallel resistive impedanceacross the discharge paths is concerned. However, Fig. 1 illustrates thegeneral case where any type of discharge path may be employed whichmight have substantial leakage current and consequently relatively lowinherent resistive impedance. With these impedance relationshipsestablished, the resistive impedance of the equivalent resistancecomprising the parallel elements I9 and |5e is of the order of one percent of the reactive impedance of the capacitor I5d. Accordingly thedistribution of the normal system or 60-cycle voltage between theterminals l0 and II across the discharge path |5a and the capacitor 2|is determined primarily by the resistive impedance of the parallelresistor elements l9 and |5e and the reactive impedance of the condenser2|. With the described relationships prevailing approximately one-tenthof the system voltage, or ten per cent in other words, will appearacross the discharge path I 5a and ninety per cent of the system voltagewill appear across the capacitor 2| Similarly the voltage appearingacross the discharge path lfia at the system voltage frequency, normally-60 cycles, it will be approximately ten per cent of that which wouldappear across the capacitor 22. In this regard it is noted that thecapacitance of capacitor I'Id which is the inherent capacitance of theair gap, is of such low value that it does not materially affect theoperation of the network either at the system voltage frequency or undersurge voltage conditions. This is similarly true in so far as the systemvoltage conditions are concerned of the capacitors lid and Hid, whichfor the values mentioned above present an ohmic impedance two hundredtimes that of the parallel resistors associated therewith and areeffectively an open circuit condition, so that substantially all of theimpedance under 60 cycle conditions is determined by the auxiliaryimpedances I9 and 20, or in certain cases as mentioned above by theequivalent resistance presented by the parallel resistors l9 and |5e inthe case of discharge path |5a, and 20 and I Be in the case of dischargepath I6a.

From the above description it will be apparent that by appropriatelyproportioning the value of the shunt resistance comprising the auxiliaryresistor l9 and the resistor |5e relative to the Capacitance of thecapacitor 2|, approximately ninety per cent of the normal system voltageapplied between the terminals l and H may be made to appear across thecapacitor 2|, with the residue ten per cent of the terminal voltageappearing across the discharge path la. If the terminal voltage issimilarly divided between the discharge path 16a and the capacitor 22,ten per cent of the normal terminal voltage will normally appear acrossthe discharge path 16a. This, of course, means that eighty per cent ofthe voltage normally applied across the terminals i0 and II will appearacross the isolating gap [1. Analyzed in a different way, the voltageacross the discharge path i1 is equal to the difference between thevoltage across the capacitor 2! and that across the discharge path lGa.It will thus be apparent that normally a minor fraction, that is lessthan fifty per cent, of the normal 60-cycle system voltage appliedacross the terminals l0 and H of the apparatus l2 appears across each ofthe two end discharge paths [5a and Mia and that a major fraction of theterminal voltage, that is more than fifty per cent, of the 60-cyclevoltage applied across the terminals l0 and H is developed across thecenter discharge path or isolating gap i1.

It will be understood by those skilled in the art that the reactiveimpedance of a capacitor varies inversely with the frequency of theapplied voltage, whereas the resistive impedance of a resistor issubstantially unaffected by changes in the frequency of the appliedvoltage. In other words, the reactive impedance of the capacitor may beexpressed as follows:

1 21rfC where Zc is the impedance in ohms due to capacitive reactance, fis the frequency in cycles per second and C is the capacitance infarads. The resistive impedance of a resistor, on the other hand, may beexpressed as follows:

where Zr is the impedance in ohms and R is the resistance in ohms. Theseexpressions indicate clearly that the reactive impedance is dependent onfrequency, whereas the resistive impedance is independent of frequency.In the system under consideration, if the frequency of the voltageacross the terminals [0 and H is increased 100 times, or in other words,if the 60-cycle frequency value is increased to 6,000 cycles per second,the reactive impedance of the capacitors 2i and 22 will fall to a valueapproximately one-tenth that of the resistive impedance of theequivalent resistors defined by the parallel resistors I8 and His in onecase, and 20 and Hie in the other case, thereby producing a reversal inthe distribution of the a plied voltage with change in frequency suchthat approximately ninety per cent of the terminal voltage at afrequency 100 times the system frequency appears across each of the enddischarge paths [5a and lBa and the remaining ten per cent appearsacross the capacitors 2i and 22. In such case the percentage of terminalvoltage appearing across the isolating gap l1 remains at eighty per centjust as was described above. At this higher frequency of 6,000 cyclesthe reactance of the capacitors d and 15d decreases substantially sothat it is no longer negligible and begins toapproach the effectiveshunt impedance provided by the elements I! and Hie in one case, and andWe in the other case.

Consequently the reactances of these capacitors must be considered as apart of the network and the effective resistance defined by the elementsl5 and 15c, and 20 and Hie become less and less important as thefrequency increases. In other words, the capacitors Hid and lid act toset a limit upon the redistribution of the applied voltage between thedischarge path l5a and the condenser 21, and between the discharge path18a and the condenser 22. For example, if the capacitance of thecapacitor I5d is one-twentieth as large as the capacitance of thecapacitor 2|, the applied voltage redistribution with increasingfrequency will approach a condition wherein ninety-five per cent of thevoltage appears across the capacitor W1 and the residue of five per centappears across the condenser 2|, and at these extremely high values offrequency of the applied voltage the impedance of the resistors l9 and152 may be completely neglected.

Lightning surges upon distribution lines result in rapidly rising lineto ground potentials which may be regarded for purposes of analysis ashaving a certain frequency characteristic. For example, it is commonpractice to subject protective equipment to test voltage surges having awave shape which rises to crest value in from one to one and one-halfmicroseconds and then falls at varying rates. In terms of frequency, therising portion of such a transient wave would represent one-quartercycle at a frequency of 250,000 cycles per second, which isapproximately 4,000 times greater than the normal system currentfrequency of cycles. It can thus be seen that the above describedvoltage distribution under surge voltage conditions is realized, since,as pointed out above, it is obtained when a voltage having a frequencyonly times greater than normal system frequency is applied.

Thus it will be apparent that the voltage across each of the two enddischarge paths |5a and 16a is increased to a major fraction of theavailable terminal voltage in response to the application of an abnormaltransient voltage across the terminals l0 and II, without decreasing thepercentage of the available terminal voltage which is developed acrossthe discharge path I! of the isolating gap. In short, the voltagesrespectively appearing across the three paths l5a, lSa and I! tend tobecome equalized and approach substantially simultaneously the appliedterminal voltage, with the end result that all three of these dischargepaths tend to spark over simultaneously. This is because, as is obviousfrom Fig. 1, when a high frequency transient occurs the ca pacitors 2|and 22 substantially become short circuits for such transients, with theresult that the discharge paths IEa, I60, and H are each connected inparallel across the terminals l0 and H. R will be understood, therefore,that by providing the auxiliary impedance network in circuit with thethree series connected discharge paths comprising primarily the shuntresistors including the auxiliary resistors l9 and 20, and

' the inherent resistances of the discharge paths I50, and 16arepresented as I56 and 5e respectively, together with the capacitors 2|and 22, that the overall time-lag in starting operation of the apparatusmay be materially reduced.

As pointed out above, the instantaneous voltage across the isolating gapI! is equal to the difference between the instantaneous voltages acrossthe capacitor 2! and the discharge path [60. This, of course, means thatunder certain transient voltage conditions the instantaneous voltagesacross the discharge path I1 may be quite low. Thus in the case wherethe rate of voltage change between the terminals l and H is such thatthe instantaneous voltage across the discharge paths |a and |6a and thecapacitors 2| and 22 are all equal, no voltage is developed acriss thedischarge path II. This will be obvious from the above discussion.However, when any one of the three discharge paths I5a, |6a and I!sparks over, its impedance instantaneously decreases to a relatively lowvalue, namely the arc drop across the path, thus causing aredistribution of the terminal voltage between the other two dischargepaths. It will be seen, therefore, that the order in which sparkover ofthe three discharge paths may occur depends, in part at least, upon thesteepness of the transient wave form of voltage applied to the terminalsl0 and I, and may be different for impressed transients of differentcharacteristics. The manner in which system current flow through thethree series connected discharge paths is interrupted after the surgecurrent is drained from the conductor |3 to ground will readily beapparent to those skilled in the art.

The advantages of the present improved system in surge protectiveapparatus may be more apparent when its operation is considered in termsof impulse ratio. This term refers to the ratio of the high frequency orimpulse voltage to the low frequency voltage required to initiate aprotective device. It is, of course, desirable that a lightning arresterafford substantially a short circuit to ground for high voltagetransients and furthermore afford substantially an open circuit toground for ordinary system voltages. Thus, impulse ratio is a truemeasure of the effectiveness of a lightning arrester. A high impulseratio obviously indicates a slow and therefore a poor impulse protectivedevice, whereas a low impulse ratio indicates an efficient one. Forexample, if it were desired to determine the impulse ratio of a standardrod gap and a standard sphere gap a surge voltage rising to its crestvalue in one and one-half microseconds and decaying to half value inforty microseconds might be applied as the impulse voltage and a quartercycle of sixty cycle voltage of sufficient magnitude to cause spark overshould also be applied. It would be discovered that the impulse ratiofor a standard rod gap would be of the order of 1.5, while for a spheregap of proper design the impulse ratio would approach unity, but alwaysmore than one. The present improved system, on the other hand, permitsimpulse ratios to be realized of less than unity which is very desirablesince it indicates that the protective device recognizes the differencebetween surges and ordinary system voltages. In the particular caseconsidered above the impulse ratio would be approximately 0.75, or abouthalf that of the standard rod gap. Consequently a device based upon thepresent invention would have an improved impulse protective level ofonehalf that of the rod gap for the same total gap spacing. Referring toFig. l of the drawings, the impulse sparkover voltage for the systemwithout the capacitors El and 22 would be about twice what it is withthe complete reversing network as shown and described above. In otherwords, a very great improvement is produced by employing the reversingnetwork of the present invention.

In the description of Fig. 1 of the drawings it was mentioned thatlightning arresters such as 10 I5 and I6 might be employed havinginherently included therein sufficient shunt resistance |5e and [Be asto supply completely the shunt resistance required for the reversingnetwork of the present invention without requiring any auxiliaryresistors such as l9 and 20. Such an arrangement is disclosed in Fig. 2of the drawings, where there is illustrated a commercial embodiment ofthe schematic arrangement shown in Fig. 1. As illustrated in Fig. 2,there is disclosed a protective evice or lightning arrester 30 which maybe in the form of a station arrester comprising a plurality of completeunits 30a, 3012, etc., the unit 30a being completely shown while only aportion of the unit 30b is illustrated. For certain voltage ratings theunit 30a may be all that is necessary, whereas for higher potentialsadditional units are added thereto in a manner well understood by thoseskilled in the art. Each unit such as 30a and 30b comprises a pluralityof separate sections 3|, 32, 33 and 34 which embody the reversingnetwork discussed above. The additional units such as the unit 3012,only a portion of which is shown, are mentioned merely to indicate thepossibility of additional units for higher voltages, but it should beunderstood that the portion 30a of the protective device 30 shown inFig. 2 between the terminals 36 and 31 thereof is completely operativeby itself and for a predetermined voltage range would comprise acomplete protective device. The terminal 31 is illustrated as the lowvoltage terminal and is schematically illustrated as being connected toground H in the same manner as the terminal H of Fig. 1. correspondinglythe terminal 36 is indicated schematically as being connected to thehigh tension transmission line |3, which in the illustrated embodimentis the protected system portion. It should be understood that theprotected system portion might be a central station, a substation or'thelike.

The terminal 31, which is the ground terminal, is connected to asuitable conducting support 39 which if only the unit 3011 is employeddefines the base of the arrester and upon which are supported; as bysuitable bolts or fastening means 40, metal castings 4| and 42, whichform parts of the sections 32 and 34 respectively. The section 32further includes an insulated cylindrical housing 43, closed at thebottom as indicated at 43a and preferably formed of porcelain or thelike, which is suitably united to the casting 4| by an insulating cement44. Preferably the casting 4| includes a flanged portion into which theclosed end of the housing 43 may be received and fastened therein by thecement 44. The section 34 also includes a porcelain housing identicalwith the housing of the unit 3| described hereinafter and fastened tothe casting 42 in a manner similar to that used in fastening the casting4| to the housing 43. The open upper ends of the housings 43 and 45 areprovided with suitable ring castings 41 and 48 respectively, suitablyattached to the housings 43 and 45, and each includes an electrodemember 49 between which is defined an isolating gap designated as IT, sothat its relationship to the gap l! in Fig. 1 of the drawings may beapparent. The electrodes 49 are adjustably mounted on the ring castings41 and 48 to permit any desired gap setting for the isolating gapSuitably supported on the ring castings 41 and 48 are insulatinghousings 50 and 5|, preferably formed of porcelain cylinders. Thehousing 50 is identical with the housing 45 except that the former isinverted and similarly the housing is identical with the housing 43 ifthe latter were inverted. As will become apparent the arrester sections33 and 34 are substantially identical with the sections 32 and 3|respectively assuming the latter sections are inverted. Attached to theupper end of the cylindri cal housings 58 and 5 l are end castings 54and 55 respectively, which are shaped to receive the upper ends ofthehousings 50 and 5| therein so as to be fastened thereto by a suitablecement indicated at 56 with reference to the housing 50 and the flange54. A suitable top conducting member 5! extends across the castings 54and 55 which castings are preferably fastened thereto by suitablefastening means 58. The top conducting member 51 comprises the highpotential end of the protective device 30 and is connected with theterminal 36.

For the purpose of relating the commercial embodiment illustrated inFig. 2 of the drawings with the schematic disclosure of Fig. 1, it ispointed out at this time that the section 3| defines a discharge pathand substantially correspohds to the discharge path I5d of Fig. l. Thesection 32 includes a condenser corresponding with the condenser 21 ofFig. 1. Similarly the section 34 includes a discharge path correspondingto the discharge path [6a of Fig. 1 and the section 33 includes acondenser corresponding to the condenser 22 of Fig. 1. These elements ofFig. 2 including the isolating gap I! as will be apparent from theensuing description, are connected into an electrical circuit identicalwith that schematically shown in Fig. 1 of the drawings.

Essentially the discharge pain associated with the section 3| of theprotective device 30 comprises an expulsion unit including an expulsiontube 60 preferably formed of insulating material capable of evolving anarc extinguishing gas when subjected to the heat of an electric arc.This expulsion tube 60 is illustrated as being internally threadedadjacent its upper andlower ends as designated at 60a and 60b in thedrawings. The upper end of the expulsion tube is completely closed bymeans of a cup-shaped cap 6| threadedly engaging with the thread 60adefined at the upper end of the expulsion tube 60. Preferably the lowerend of the expulsion tube 60 which is concentrically disposed within thehousing 50 rests upon the central casting 41 which is provided with acentral opening through which extends a suitable conducting tube 62having its upper end threadedly engaged with the thread 605 and havingan inner bore coextensive with the bore within the expulsion tube 60. Asuitable transverse conducting rod 63 defining the lower terminal of theexpulsion arrester extends transversely across the lower end of theexpulsion tube and electrically connects the ring casting 41 thereto sothat the external isolating gap I1 is electrically connected to oneterminal of the discharge path corresponding to the path a in Fig. l ofthe drawings.

For the purpose of defining the spark over gap and also providing meansfor aiding in rapidly extinguishing the arc drawn there are disposedwithin the bore of the expulsion tube 60 a pair of filler rods 64 and 65arranged in end to end relationship. Only two of these rods are shown inFig. 2 of the drawings, but it will be understood that additional rodsmay be employed or disposed in end to end relationship to give thedesired expulsion characteristic. The lower filler rod 65 is best shownin Fig. 3 of the drawings and comprisesa rod of gas evolving insulatingmaterial such ashard fibre or the like having a tubular metal ferrule 66at the upper end thereof and a ferrule 61 at the lower end thereof. Theends of the filler rod are of reduced cross section so as to be receivedwithin the ferrules 66 and 51. The ends of the ferrules 66 and 61 arefurthermore out at an angle designated as 66a and 61a respectively. Theupper ferrule 66 is of tubular configuration so that a portion 68 of theinsulating filler rod 65 extends beyond the end of the ferrule 66 as isclearly shown in the drawings. This end is also cut at the same angle asthe corresponding end of the ferrule.

' Toprovide a short spark over gap the lower ferrule 61 is provided withan integral extension 69 embedded in the surface of the filler rod 65 asshown so as to provide a fairly short are gap between the upper endthereof and the ferrule 66. The conductor 69 and the initial arc pathdefined between its upper end and the ferrule 66 are so arranged withrespect to the angular surfaces 66a and 61a that the maximum lengthsalong the longitudinal axis of the ferrules 66 and 6! are colinear withthe conductor 69. The filler rod 64 is substantially identical with thefiller rod 65 except that the ferrule at the lower end thereof isidentical with the ferrule 66 of the filler rod 65 and is designated bythe same reference numeral. These filler rods 64 and 65 areof somewhatsmaller diameter than the bore of the tube 60 so that when disposed inthis bore a crescent shaped chamber is defined between the filler rodand the tube bore.

In order to increase the arc interrupting ability the filler rods arebiased to cause the portions thereof colinear with the conductors 69 toengage the walls of the bore of the tube 60. To this end the angular endsurface 61a of the lower for rule 61 engages with the transverseconductor 63 at the lower end of the expulsion tube 60 to force thefiller rod 65 away from the center of the tube60 so that the conductor69 bears against the wall of the expulsion tube 60. The upper filler rod64 is inserted so that the interengaging angular end surfaces of theupper and lower filter rods are in the position shown in Fig. 2 with theinsulating extensions 66 in engagement. The purpose of the insulatingextensions 68 at the interengaging faces of the filler rods 64 and 65 isto prevent nietal to metal contact between the adjacent ferrules such as66 so as to prevent welding together of these ferrules under the heat ofthe arc. I For the purpose of aiding in positively forcing the conductorportions 69 on each filler rod into intimate engagement with the wallsof the bore of the expulsion tube 60 there is provided a suitablecoinpression spring 10 disposed within the cup-shaped cap 6| compressedbetween the upper end of the filler rod 64 and the top of the cap 6|.Preferably there is interposed an adapter member 1| between the lowerend of the compression spring!!! and the upper end of the filler rod 64to permit proper cooperation between this end of the compression springand the upper angular end of the filler rod 64.

In order to connect the upper terminal of the expulsion device describedabove with the line terminal 35 there is provided a suitable conductingplate I5 supported within the upper casting 54 by fastening meansillustrated at 16. A suitable spring member 1T interposed between theplate 15 and the closure cap 6! of the expulsion tube 60 completes theelectrical circuit from the terminal 36 to the upper terminal of theexpulsion discharge path, and also maintains the expulsion tube 68 inproper position with respect to the ring casting 4'1. The expulsionarrester described in detail above forms no part of the presentinvention but is disclosed and claimed in a Pittman Patent 2,434,010.

The operation of the expulsion device described above will be understoodby those skilled in the art. In the event of a surge voltage appearingon the conductor I3 of sufiicient magnitude to initiate a dischargebetween the transverse conducting rod 63 and the upper ferrule 66 of thefiller rod 64, there first are formed a series of arcs, one between theupper ferrule 66 of the filler rod 64 and the upper end of the conductor69 of that filler rod, another between the upper ferrule 66 of the lowerrod 65 and the conductor 69 of that filler rod, and still another shortarc between an upper ferrule 66 of the lower filler rod 65 and lowerferrule 66 of the upper rod 64. These initial arcs and particularly thetwo long are sections thereof will be initiated in the most confinedspace between the filler rods and the expulsion tube walls by virtue ofthe action of the compression spring 18 tending to bias these fillerrods against different wall portions of the expulsion tube 60. Theimmediate production of arc extinguishing gases which are evolved fromthe walls of the expulsion tube 68 and the filler rods 64 and 65 causesthe arcs to be transferred to the larger space between the filler rodsand the expulsion tube walls opposite the conductors 69. The combinedaction of the arc extinguishing gas and the arc elongation tends rapidlyto extinguish the arc in the conventional manner of an expulsion tube.The are discharge products are exhausted through the lower end of theexpulsion tube 66.

With the arrangement described thus far, it will be understood that aregases escaping from the lower end of expulsion tube 66 might tend tocause a flash over around the outside of the expulsion device. Toprevent this the annular space between the expulsion tube 66 and thehousing 50 is filled with an arc cooling material of some sort.Preferably a refractory material having the desired resistivity such asgranular silicon carbide generally designated at 18 is employed to servea dual function of cooling the escaping are a spark over of thedischarge device described above. To insure proper cooling of theescaping arc gases and proper dififusion through the mass M ofgranularsilicon carbide 18, the conducting tube 62 may extend substantially tothe bottom of the housing 43 and be provided with suitable openings suchas saw cuts or other openings or perforations. As illustrated in thedrawings, however, a suitable tubular extension 19 for the conductingtube 62 is defined by a perforated screen like material having a meshsuch as to require the arc gases to escape throughout substantially theentire surface thereof. ment the entire annular space in the lowerhousing 43 as well as the upper housing 56 may be filled with granularsilicon carbide thereby increasing the path length through which the arcgases may be diffused, The ring casting 41 is preferably provided with aplurality of perforations 86 to provide a path for are gases from thelower to the upper annular chamber. In order to maintain the granularsilicon carbide mass in the desired compressed condition there isdisposed With this arrangeadjacent the upper end of the housing 50 anannular piston like member 83 which is forced downwardly by acompression spring 84 whose upper end bears against the plate 15. Asuitable downwardly directed vent to atmosphere 86 is associated withthe upper casting 54 to provide a suitable escape passageway for aregases released through the gas cooling mass 18.

In accordance with the present invention there is disposed within thelower end of the housing 43 a cup-shaped conducting member 88 whichtogether with the adjacent surface of the casting 4| defines a capacitorhaving a porcelain dielectric comprising the portion 43a of the housing43 disposed between the capacitor plates. The capacitance definedbetween the member 88 and 4| corresponds to the capacitor 2| in Fig. 1of the drawings. To insure positive electrical connection between thecondenser plate 88 and the lower electrode of the expulsion typedischarge path described above there is provided a resilient member 88formed of conducting material which is compressed between the end of theextension 19 of the tubular member 62 and the condenser plate 88.

Although the discharge path within the housing 50 essentially comprisesan expulsion arrester, by employing in shunt therewith the siliconcarbide granules for gas cooling purposes there is effectively provideda shunt resistor which corresponds to the resistor I5e in Fig. 1 of thedrawings since it is an inherent part of the arrester unit. This shuntresistance formed of granular silicon carbide furthermore functions as avalve member in shunt with the expulsion discharge path to by pass surgecurrents when the surges are insumcient to cause breakdown of theexpulsion arrester. The valve action is such as to limit the flow ofpower follow current.

In view of the detailed description included above, the operation of theprotective device 30 in Fig. 2 of the drawings, which has been shown tobe basically identical with Fig. 1 of the drawings, will be understoodby those skilled in the art. Since the silicon carbide granules whichare an inherent part of the arresters employed in the protective device36 and correspond to the resistors l5e and l6e of Fig. l of thedrawings, provide a low shunt resistance path no auxiliary resistorssuch as 19 and 20 may be required, assuming that this shunt resistancepath provides the desired resistance for the proper functioning of thereversing network. When properly designed, under normal system voltageconditions the major portion of the system voltage appears across theplates of the capacitor defined by the capacitor electrodes 88 and 4|. Aminor portion, and preferably less than 10 per cent voltage appearsacross the discharge paths defined within the housings 50 and 45 whereasabout 80 per cent of the system appears across the discharge path I1. Inthe event of a surge voltage condition however, the potential reversingfunctin of the network comes into effect with the result that a majorportion of the surge voltage appears across the discharge paths toinitiate a discharge through the expulsion device if the surge 15 ofsuincient magnitude or through the granular silicon carbide in the eventof a surge of lower magnitude. In either case the arrester 15 providedwith greatly improved impulse char acteristics.

In the event that the inherent shunt resistance of arrester units isinsuflicient to give the desired voltage distribution under normal andsurge conditions auxiliary resistors 19 and 20 may readily be providedwith the arrangement disclosed in Fig. 4 of the drawings, by providingthe filler rods 64 and with cores of resistance material to provide theshunt resistance path of the desired resistance value. In Fig. 4 of thedrawings there is illustrated a portion of the sections 31 and 32 ofFig. 2 to illustrate this modification of the invention. Thecorresponding parts of Fig. 4 are designated by the same referencenumerals as in Fig. 2 of the drawings. In this case each of the fillerrods designated as 64 and 65' are identical with those in Fig. 2 of thedrawings except for the fact that each is provided with a center core 90formed of a suitable resistance material to provide a predeterminedresistance across the ends of the respective filler rods. These coresfurthermore extend beyond the ends of the filler rods 64 and 65 asindicated in Fig. 4 of the drawings to provide extensions 90afunctioning just like the extensions 63 in the arrangement disclosed inFig. 3 of the drawings, except that these extensions 90a are formed ofresistance material to provide a continuous resistance path through bothfiller rods 64' and 65.

In Fig. 4 the expelled arc gases from the expulsion tube 88 are notpermitted to be dispersed through the granular silicon carbide, andaccordingly, attached to the lower end of the expulsion tube is atubular member 95 filled with suitable baiiiing material 96 to aid incooling the arc gases. A suitable vent to atmosphere may be provided topermit the arc gases to escape after being suitably cooled. Under theseconditions obviously no filler material need be provided within thelower housing 43 and the ring casting designated as 41 is constructed soas to seal off the annular space between the expulsion tube 60 and thehousing unit 50 from the lower section 32. This space above the ringcasting ll may again be filled with a mass of valve material such asgranular silicon carbide to provide a discharge path for surgesinsufiicient to cause a spark over within the expulsion device. Theresistance cores 90 of the filler rods 64 and provide an auxiliaryresistance corresponding to the element I9 in Fig. 1 which combined withthe inherent resistance provided by the granular silicon carbide inshunt therewith determines the effective shunt resistance. In the eventthat the granular silicon carbide is omitted from Fig. 4 the resistancecores provide substantially the entire shunt resistance.

While certain particular embodiments of the invention have been shown,it will be understood, of course, that the invention is not limitedthereto since many modifications may be made, and it is contemplated bythe appended claims to cover any such modifications as fall within thetrue spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. Protective apparatus comprising a pair of terminals across which arelatively low frequency alternating current voltage is normallyimpressed and across which an abnormal surge voltage may occasionally beimpressed, means providing a plurality of discharge paths connected inseries between said terminals, auxiliary impedance means connected incircuit with said discharge paths including a resistance shunting eachend discharge path and a capacitor connecting each end discharge pathacross said terminals, said resistance having such a value of impedancerelative to the remainder of said auxiliary impedance means for causingonly a minor fraction of said low frequency voltage across saidterminals to appear across said end discharge paths.

2. Protective apparatus comprising a pair of terminals adapted to have anormal low frequency alternating current voltage impressed thereacrossbut occasionally subject to abnormal transient voltages, means providinga plurality of discharge paths connected in series between saidterminals, a static impedance means associated with one of saiddischarge paths comprising one impedance independent of frequencyconnected across said one discharge path and another impedance dependenton frequency connected in series with said one impedance across saidtermi nals, said one impedance having such a value relative to saidother impedance at normal low frequency alternating currents to causethe voltage across said one of said discharge means to comprise a minorfraction of said low frequency alternating current voltage and a majorfraction of said abnormal transient voltage.

3. Protective apparatus comprising a pair of terminals adapted to havean alternating system voltage and abnormal transient voltages impressedthereacross, a pair of expulsion type lightning arresters and anisolating gap connected in series between said terminals with saidisolating gap connected between said arresters, capacitance meansconnecting each of said arresters directly across said terminals, and aresistive impedance shunting each of said arresters to coact with saidcapacitance means in normally maintaining only a minor fraction of thesystem terminal voltage across each of said arresters, said capacitancemeans co-acting with said resistive impedances to increase the voltageacross each of said arresters to a major fraction of the terminalvoltage when a transient voltage appears between said terminals.

4. Protective apparatus comprising a pair of terminals adapted to havean alternating system voltage and fast rising transient voltagesimpressed thereacross, a pair of expulsion type lightning arresters andan isolating gap connected in series between said terminals with saidisolating gap connected between said arresters, capacitance meansconnecting each of said arresters directly across said terminals, and aresistive impedance shunting each of said arresters, the resistiveimpedance shunting each arrester being, at the system voltage frequency,a minor fraction of the capacitive impedance of the capacitance meansconnecting the associated arrester across said terminals and the latterimpedance being, at the system voltage frequency, a minor fraction ofthe capacitive impedance of the associated arrester, whereby a minorfraction of the system terminal voltage normally appears across each ofsaid arresters and a major fraction of said system terminal voltagenormally appears across said gap and whereby the decrease in saidcapacitive impedances relative to said resistive impedances resultingfrom the appearance of a fast rising transient voltage between saidterminals causes a major fraction of the terminal transient voltage toappear across each of said arresters without substantially decreasingthe fraction of the terminal voltage appearing across said gap.

5. Protective apparatus comprising a pair of terminals adapted to havean alternating system voltage and steep wave front transient voltagesimpressed thereacross, means providing 17 three discharge pathsconnected in series between said terminals, each of the two enddischarge paths having inherent shunt capacitance and shunt resistance,the ohmic reactance of the capacitance of each end discharge path beinghigh relative to the shunt resistance at the system voltage frequencyand the reactance of the capacitance of each end discharge path beinglow relative to the shunt resistance at the rate of .voltage change of afast Wave front transient voltage, and a capacitor connecting each ofsaid end discharge paths directly between said terminals, the capacitorassociated with each end discharge path having a capacitance exceedingby several times the capacitance of the associated end discharge path,whereby a major fraction of the terminal voltage normally appears acrosseach of said capacitors and the center discharge path, a minor fractionof the terminal voltage normally appears across each of the enddischarge paths, and a voltage exceeding a major fraction of a steepwave front transient voltage appearing between said terminals isdeveloped across each of said discharge paths each time a steep Wavefront transient voltage appears across said terminals.

6. Protective apparatus comprising a pair of terminals adapted to have anormal voltage and abnormal transient voltages impressed thereacross,means providing a plurality of discharge paths connected in seriesbetween said terminals, capacitor means for connecting each pathdirectly across said terminals, and a reversing network comprising saiddischarge paths, capacitor means and static impedance means, said staticimpedance means being connected in parallel with at least one of saiddischarge paths whereby said reversing network is responsive to atransient voltage between said terminals to increase from a minor ratioto a major ratio the magnitude of the voltage across each end dischargepath as related to the normal voltage across said terminals.

'7. The protective system comprising a pair of terminals adapted to havea normal voltage and abnormal transient voltages impressed thereacross,means providing a plurality of discharge paths connected in seriesbetween said terminals, means including a resistance shunted across eachend discharge path having such a value of impedance relative to thetotal impedance across said terminals for normal voltage that the normalvoltage applied to said terminals is unequally distributed across saidpaths, and means including said last mentioned means responsive to atransient voltage between said terminals for reducing the inequalitybetween the voltages across said paths.

8. Protective apparatus comprising a pair of terminals adapted to havean alternating system voltage and abnormal transient voltages impressedthereacross, means providing three discharge paths connected in seriesbetween said terminals, the two end discharge paths comprising resistiveand capacitive impedances in parallel, and capacitance means connectingeach of the end discharge paths directly across said terminals, thecapacitances of said capacitance means being so proportioned relative tothe resistive and capacitive impedances of said end discharge paths thata minor fraction of the terminal voltage normally appears across each ofthe end discharge paths and a major fraction of a transient voltageappearing between said'terminals is developed across each of the threedischarge paths.

9. Protective apparatus comprising a pair of terminals, a pair ofcurrent paths bridging said terminals in parallel and each seriallyincluding a capacitor and a discharge gap having shunt resistance andcapacitance, said paths being oppositely connected between saidterminals so that one path includes its capacitor adjacent one terminaland the other path includes its capacitor adjacent the other terminal,and means defining at least one additional gap bridging the junctionpoints between said first-named gaps and their respective associatedcapacitors.

10. A protective device comprising an insulating supporting structureincluding a cylindrical housing, a pair of terminals mounted on saidstructure, an expulsion type discharge path disposed in said housingelectrically connected between said terminals, and gas cooling meansdisposed in said housing for cooling arc gases developed upon operationof said expulsion type discharge path, said gas cooling means alsoinherently providing a surge discharge path in shunt with said expulsiontype discharge path.

11. A protective device comprising an insulating supporting structureincluding a cylindrical housing, a pair of terminals mounted on saidstructure, an expulsion type discharge path disposed in said housingelectrically connected between said terminals, and gas cooling meanscomprising a mass of granular silicon carbide disposed in said housingfor cooling arc gases developed upon operation of said expulsion typedischarge path, said granular silicon carbide being disposed in saidhousing to provide a surge discharge path in shunt with said expulsiontype discharge path.

12. A protective device comprising an insulating supporting structureincluding a plurality of cylindrical housing sections, a pair ofterminals mounted on said structure, an expulsion type discharge pathelectrically connected to one of said terminals and disposed in one ofsaid housing sections so as to define an annular chamber in said onehousing section, means electrically connecting said discharge path tosaid other terminal, and gas cooling means within said chamber forcooling arc gases developed upon operation of said expulsion typedischarge path, said gas cooling means comprising a resistance materialproviding a path for surge currents in shunt with said expulsion typedischarge path.

13. A protective device comprising an insulating supporting structureincluding a plurality of cylindrical housing sections, a pair ofterminals one mounted adjacent each end of said structure, meansdefining an air gap disposed outside said housing sections between saidterminals, an expulsion type discharge path disposed Within one of saidhousing sections and electrically interconnected between one of saidterminals and one electrode of said air gap, a conducting memberdisposed in another of said housing sections, and defining with meansconnected with the other of said pair of terminals a capacitor, saidconducting member comprising one plate of said capacitor, meansconnecting said one plate of said capacitor to said one electrode ofsaid air gap, and gas cooling means within said housing sections forcooling arc gases developed upon operation of said expulsion typedischarge path, said gas cooling means comprising granular siliconcarbide and providing a path for voltage surges in shunt with saidexpulsion type disclosure path.

14. A protective device comprising an insulating supporting structureincluding a cylindrical housing, a pair of terminals mounted on saidstructure, an expulsion type discharge path disposed in said housingcomprising an expulsion tube and a pair of electrodes disposed thereinand electrically connected between said terminals, an arc confiningfiller rod formed of gas evolving insulating material disposed in saidexpulsion tube between said terminals, and a resistance core in saidfiller tube for providing a resistance in shunt with said dischargepath.

EDWARD H. YONKERS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,171,166 Brach Feb. 8, 19161,545,646 Everett July 14, 1925 1,902,510 McEachron Mar. 21, 19332,330,918 Pittman Oct. 5, 1943 2,338,479 Ackerman "-1 Jan. 4, 19442,434,010 Pittman Jan. 6, 1948 FOREIGN PATENTS c Number Country Date40,108 Switzerland Apr. 22, 1907 36,708 Austria Mar. 26, 1909

